CHAPTER LXXI. EXAMINATION OF LAKES.

CHAPTER LXXI.
EXAMINATION OF LAKES.

In examining lakes it is necessary to ascertain the nature of the organic colouring matter, the base with which this is united, and also the nature of the substances mechanically mixed with the lake to lighten its shade.

It is most convenient to examine the pigment first for mechanical admixtures. If an effervescence follows the addition of hydrochloric acid, denoting the presence of a carbonate, this is generally calcium carbonate. White lead is rarely used, since it is very heavy and is more expensive. Gypsum and magnesia are occasionally added to lakes; they are recognised by examining the residue left on ignition.

In testing for the metallic oxide which is united with the colouring matter a small quantity of the lake is heated in a little porcelain crucible until all organic substances are completely destroyed. The residue is then examined for aluminium and tin oxides, by means of which lakes are most frequently prepared.

Reactions of the Organic Colouring Matters.

Colouring Matter.	Dilute Acids.	Caustic Soda.	Lime Water.	Strong Nitric Acid.	Alum.
YELLOW.
Buckthorn berry extract	Slight turbidity.	Yellowish red.	Slight greenish precipitate	Reddish brown.	Pale yellow.
Dutch pink	Yellow solution.	Brown.	Brownish	Reddish brown.	——
Weld extract	Slight turbidity.	Golden yellow.	Greenish yellow.		Slight yellow precipitate.
Weld lake	Yellow solution.	Unaltered.	Greenish yellow.	Golden yellow solution.	——
Gamboge	Unaltered	Deep reddish yellow.	Deeper yellow.	Deep yellow.	——
Gamboge lake	Yellow solution.	Deep reddish yellow.	Deeper yellow.	Deep yellow.	Yellow precipitate.
Fustic	Becomes paler.	Dirty brownish red.	Deeper.	Slight precipitate.	Pale yellow precipitate.
Quercitron	Becomes paler.	Deeper.	Deeper.	Paler.	Paler, slight precipitate.
Young fustic	Slight greenish precipitate.	Purple red.	Reddish.	Greenish.	Slight greenish precipitate.
Turmeric	Carmine red.	Brown.	Brown.	Carmine red.	Yellow.
BLUE.
Indigotin	Unaltered.	Yellowish brown solution.	——	Yellow.	——
Indigo carmine blue lake	Unaltered.	Yellowish brown solution.	——	Yellow.	——
Litmus	Turns red.	Unaltered.	——	——	——
Logwood	Red.	Violet.	Blue.	Yellow.	Dirty violet.
RED.
Cochineal decoction	Becomes paler.	Violet.	Becomes darker.	Yellow.	Red flocks.
Carmine	Red solution.	Violet solution.	Becomes darker.	Yellow.	——
Lac, lac dye	Yellowish red.	Violet solution.	——	Yellow.	Red precipitate.
Safflower red	Unaltered.	Red solution.	——	——	——
Madder lake	Red solution.	Red precipitate.	——	Yellow.	Red precipitate.
Red wood	Pale red.	Violet.	Carmine red precipitate.	Yellow.	Red precipitate.
Red wood lake	Red solution.	Violet solution.	——	Yellow.	——
The remaining red colouring matters give such uncertain reactions that it is almost impossible to distinguish between them by ordinary means; they are therefore not included here.
GREEN..
Sap green	Reddish brown.	Brownish green.	Becomes deeper.	Reddish yellow.	——
Leaf green	Green solution.	Green solution.	——	Reddish yellow.	——
BROWN..
Sepia	Unaltered.	Brown solution.	——	——	——
Catechu solution	Paler.	Reddish brown.	Reddish brown.	Yellowish brown.	Reddish yellow coloration.

Colouring Matter.	Stannous Chloride.	Ferric Chloride.	Copper Acetate.	Gelatine Solution.	Remarks.
YELLOW.
Buckthorn berry extract	Slight greenish yellow precipitate.	Olive green.	Dirty yellow.	Slight precipitate.	——
Dutch pink	——	——	——	——	Chars when heated on platinum foil.
Weld extract	Considerable yellow precipitate.	Greenish	Reddish yellow precipitate.	Slight turbidity.	Gives yellow scales some time after adding potassium bichromate.
Weld lake	——	——	——	——	——
Gamboge	——	——	——	——	Readily melts and burns.
Gamboge lake	Yellow precipitate.	——	——	——	——
Fustic	Yellow precipitate.	Blackish green precipitate.	Brownish yellow precipitate.	Considerable precipitate.	Lead acetate gives an orange red precipitate.
Quercitron	Yellowish red precipitate.	Becomes green, green precipitate.	Yellowish green precipitate.	Reddish precipitate.	——
Young fustic	Orange red precipitate.	Brownish green precipitate.	Brown flocculent precipitate.	Yellowish red precipitate.	Ammonia gives red coloration.
Turmeric	Yellow.	Brown.	——	Fairly large precipitate.	Very sensitive to alkalis.
BLUE.
Indigotin	——	——	——	——	On heating gives peculiar odour and purple red vapours.
Indigo carmine blue lake	——	——	——	——	On heating gives peculiar odour and purple vapours.
Litmus	——	——	——	——	Unpleasant smell on heating, white ash.
Logwood	Dirty violet.	Dark coloured solution.	——	Very turbid.	Black with potassium bichromate.
RED.
Cochineal decoction	Reddish violet flocks.	——	——	——	Completely soluble in ammonia.
Carmine	——	——	——	——	——
Lac, lac dye	Red precipitate.	——	——	——	——
Safflower red	——	——	——	——	Rapidly decomposed by boiling with alkalis.
Madder lake	Red precipitate.	——	——	——	——
Red wood	Deep red precipitate.	Violet precipitate.	Violet red precipitate.	——	——
Red wood lake	——	——	——	——	Much ash on ignition.
The remaining red colouring matters give such uncertain reactions that it is almost impossible to distinguish between them by ordinary means; they are therefore not included here.
GREEN..
Sap green	——	——	——	——	White ash on ignition.
Leaf green	——	——	——	——	White ash on ignition.
BROWN..
Sepia	——	——	——	——	Burns with peculiar smell.
Catechu solution	Brownish precipitate.	Brownish green precipitate.	Brown precipitate.	Considerable precipitate.	——

It is not easy to decide which colouring matter is united with the metallic oxide, because the organic colouring matters do not give, as a rule, decided reactions. In examining pigments of this kind it is always advisable to test at the same time a pigment of known composition, and to compare the reactions of the two. The colouring matter of the lake is first brought into solution. This is accomplished by the action of dilute hydrochloric acid, which decomposes the compound of metallic oxide and colouring matter, the latter dissolving. A portion of the lake should remain undecomposed, so that the solution has no strong acid reaction by which the action of the reagents to be used would be modified. When the lake has been treated for some time with dilute hydrochloric acid, water is added and the solution of the colouring matter filtered from the residue. Small quantities of this solution are then treated with the different reagents. By a comparison of the reactions given by the solution and by a solution of the pure colouring matter, the nature of the colouring matter under examination may be decided.

In deciding in the first place if a colour is of organic origin, it is treated with hydrochloric acid in the above manner. If a coloured solution results, an organic colour is probably present. Chlorine water is then added to a portion of the solution; if the latter is quickly decolourised an organic colouring matter is certainly present, for they are all decomposed by the continued action of chlorine.

The following reagents are used in testing organic colouring matters: dilute sulphuric or hydrochloric acid, caustic soda solution or lime water, strong nitric acid, which, in consequence of its oxidising properties, gives different reactions to the other acids. Of the metallic salts, alum, stannous chloride and ferric chloride are used, and occasionally copper acetate. Glue solution is also used in the pure form of isinglass or gelatine solution, with which several colouring matters give characteristic precipitates. The foregoing tables give the behaviour of the colouring matters contained in the organic pigments towards the reagents mentioned above.